linux-kernel - Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch

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Message-ID: <CA+55aFztN+nGBnb1Wjvygvgkg2MfN9m88egsostaNJ5LoSQx7A@mail.gmail.com>
Date:   Mon, 8 Jan 2018 16:35:31 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on
 context switch

On Mon, Jan 8, 2018 at 3:58 PM, Woodhouse, David <dwmw@...zon.co.uk> wrote:
>>
>> Is there really nothing more clever we can do?
>
> You get this part in the IBRS/microcode solution too. The IBRS MSR
> doesn't catch everything; you still need to stuff the RSB in very
> similar places (and/or use the IBPB MSR in some).

So I was really hoping that in places like context switching etc, we'd
be able to instead effectively kill off any exploits by clearing
registers.

That should make it pretty damn hard to then find a matching "gadget"
that actually does anything interesting/powerful.

Together with Spectre already being pretty hard to take advantage of,
and the eBPF people making those user-proivided gadgets inaccessible,
it really should be a pretty powerful fix.

Hmm?

              Linus